It is known to disconnect a vehicle driveline and to reduce an operating speed of an internal combustion engine to reduce fuel consumption. This strategy is known variously as vehicle gliding, vehicle sailing, idle coasting etc. The operating mode is referred to herein as a gliding mode.
An example of a known driveline disconnect strategy is disclosed in the Applicant's earlier UK patent application GB1316183.1. A rear-wheel drive vehicle 1 having a powertrain 3 is illustrated in FIG. 1. The powertrain 3 comprises an internal combustion engine 4, a transmission 5 and a driveline 6. When a gliding mode is activated, the driveline 6 is decoupled from the internal combustion engine 4. The operating speed of the internal combustion engine 4 can then be reduced, for example to operate at idle, to provide improved fuel efficiency. When the internal combustion engine 4 is decoupled, the driveline 6 is rotated by a torque applied by the driven wheels WD (the rear wheels in the present arrangement). The dynamic operating states of the respective components when the vehicle 1 is operating in a conventional gliding mode are illustrated in FIG. 1.
A vehicle 1 having a front-wheel drive arrangement is illustrated in FIG. 2. The front-wheel drive vehicle 1 can also operate in a gliding mode by decoupling the driveline 6 from the internal combustion engine 4. When the driveline 6 is decoupled, the driveline 6 is rotated by a torque applied by the driven wheels WD (the front wheels in the present arrangement). The dynamic operating states of the respective components when the vehicle 1 is operating in a conventional gliding mode are illustrated in FIG. 2.
The relationship between the operating loads on a vehicle 1 travelling down a 2% negative gradient is illustrated in FIG. 3. The loads are expressed as the torque within a powertrain of the vehicle 1. The positive (accelerating) forces acting on the vehicle 1, represented by a first arrow pointing in the direction of travel (from left to right in FIG. 2), comprise: an engine torque A delivered in dependence on a driver torque request; and an effective torque B derived from the road gradient. The sum of the engine torque A and the effective torque B represents a total torque at the wheels of A+B. The negative (decelerating) forces acting on the vehicle 1, represented by a second arrow pointing in the opposite direction (from right to left in FIG. 2), comprise: an aerodynamic torque C; a road loss torque D; an engine loss torque E; a transmission loss torque F; and a driveline loss torque G. The total negative torque is −(C+D+E+F+G); and the total positive torque is (A+B). A first difference between the positive torque and the negative torque is calculated as follows: (A+B)−(C+D+E+F+G). When the gliding mode is activated, the vehicle driveline 6 is disconnected from the internal combustion engine 4 and the total torque comprises a positive torque comprising the effective torque B; and a negative torque comprising the aerodynamic torque C, the road loss torque D, the transmission loss torque F and the driveline loss G. The internal combustion engine 4 is disconnected from the driveline 6 so the engine loss torque E is not applied. A second difference between the positive torque and the negative torque is calculated as follows: (B)−(C+D+F+G). The internal combustion engine can operate at a lower speed, for example idle, or can be switched off.
It would be advantageous to broaden the range of operating conditions in which the driveline could be decoupled from the internal combustion engine. It is against this backdrop that the present invention has been conceived.